1. Field of the Invention
The present invention relates generally to the fields of radiography and tomography. More particularly, this disclosure describes how a standard x-ray source may be used to create phase-enhanced computed tomography (CT) images. These phase-enhanced CT images provide sharper images which make small edges, including small masses or small spiculations, more evident. This provides a method for, among other things, the inspection of organic materials and for use in medical apparatuses.
2. Description of Related Art
In the conventional x-ray transmission imaging system, the contrast of an obtained image depends upon the degree of absorption of x-rays by an object. Namely, if there is a region where heavy elements with high x-ray absorptance are dense, that portion exhibits a low transmittance and can be caught as a shadow in an image. On the other hand, objects made of light elements (soft tissues, etc.) are transparent to x-rays and are therefore difficult to obtain an image contrast. For example, in many clinical situations, such as mammography, there is a need to distinguish between different kinds of soft tissue, between tumors and normal tissue, for instance. Accordingly, there is employed a method in which the contrast is emphasized by injecting heavy elements as a contrast agent in order to observe biological soft tissues (such as internal organs, tumors, or blood vessels), for example, when an x-ray cross section image for medical diagnosis is to be acquired. However, the contrasting technique cannot always be applied to all observation locations to be investigated. Also, the contrasting process may negatively impact the body.
The above problem concerning the image contrast exists similarly even in x-ray computerized tomography (CT) which is a three-dimensional, inside-observing technique. A CT scanner x-rays the body from many angles. The x-ray beams are detected by the CT scanner and analyzed by a computer. The computer compiles the images into a picture of the body area being scanned. These images can then be viewed on a monitor or reproduced as photographs.
In lieu of x-ray absorption techniques, there is an imaging method available for obtaining a contrast by monitoring x-ray phase shifts. A phase-contrast x-ray records information from the x-ray beams after they have passed through different materials, including biological soft tissues. All tissues cause the x-rays to slow down, resulting in what is known as a phase shift. The size of the shift depends on the type of tissue. Phase-sensitive techniques, which can be understood using wave optics rather than ray optics, offer ways to augment or complement standard absorption contrast by incorporating phase information. Also, since phase-contrast relies only on refraction of x-rays, not on absorption, imaging can be done at higher energies where the absorbed radiation dose can be less, thereby reducing potential damage to tissues.
The use of phase-contrast imaging methods enables observation with an excellent sensitivity which is one-thousand times as high as that in the conventional absorption-contrast method. Thus it may be possible to observe phase contrast when absorption contrast is undetectable. Furthermore, biological soft tissues can be observed without being subjected to a specific contrasting process. Also, even if a contrast agent is used, the choice of a wider variety of contrast agents and contrasting techniques are possible.
There has been devised a phase-contrast x-ray CT apparatus in which phase contrast is introduced to the x-ray CT enabling three-dimensional observation. U.S. Pat. No. 5,173,928 is specifically incorporated herein by reference in its entirety. According to the disclosed technique of the '928 patent, an x-ray interferometer is used to reconstruct an image in a virtual-cross section from interference patterns. This disclosed technique is limited to observing an object having a diameter of only several millimeters, and therefore is not practical for medical diagnosis. Further problems with the above system using the x-ray interferometer is that (1) in order to ensure coherency, the energy band of an x-ray beam must be narrowed to obtain a high monochromaticity and hence a bright light source such as synchrotron radiation must be used; and (2) that a precision optical system is required and it is therefore difficult to handle the system.
There has also been devised a phase contrast x-ray CT apparatus in which an x-ray interferometer is not used. U.S. Pat. No. 5,715,291 is specifically incorporated herein by reference in its entirety. In the '291 patent, the phase distribution is determined from the distribution of refraction angles of x-rays transmitted through an object. However, this disclosure is limited to object rotation to acquire the necessary data to reconstruct a CT image.
In view of the shortcomings such as those listed above, improved phase-contrast imaging techniques would be desirable.